A device powered by rechargeable batteries may include several battery cells to achieve voltage and/or current levels used by the device. For example, if a rechargeable battery cell has a nominal output voltage of 1 Volt, a device having a 2 Volt operational level may include two battery cells coupled in series. In another example, if a rechargeable battery cell has a nominal output current of 100 milliamps, a device having a 400 milliamp operational level may include four battery cells coupled in parallel. Battery cells coupled in parallel and series may be combined to reach the desired operational voltage and current levels of the device.
The battery cells may be grouped with circuitry for balancing the charge levels in the battery cells to form a battery pack system module. Multiple battery pack system modules may be combined in series or parallel to further increase the output voltage and output current available to a device coupled to the battery pack system modules. Battery cells within a battery pack system module may be balanced by using balancing circuitry within the battery pack system module (referred to as intra-module balancing). Battery pack system modules may also be balanced to other battery pack system modules (referred to as inter-module balancing).
Balancing battery cells is an important process for maintaining the health of and correct operation of a battery system comprising multiple battery pack system modules. The battery system may not behave as desired when battery pack system modules are out of balance with each other or battery cells within a battery pack system module are out of balance with each other. For example, an output voltage from and/or capacity in a battery system with unbalanced battery pack system modules or battery cells may be outside a desired range.
Battery cells and battery pack system modules may experience different wear and use patterns. When battery cells and battery pack system modules experience different wear and use patterns they become more out of balance with other battery cells and battery pack system modules. Battery cells or modules may become out of balance with other cells and modules for a variety of reasons.
One cause may be differences in the age of the cells or modules. For example, when old cells or modules are replaced with new cells or modules, the cells or modules may have different capacities. This results because older cells or modules typically have lower capacity. Capacity of battery cells and modules usually uniformly reduces with age until the cell or module has reached end of life.
Another cause may be difference in the capacity of the cells or modules when they are manufactured. The cells or modules may be manufactured by different suppliers with different material sources and different standards. Thus, each module or cell may have a different capacity and a different capacity decay rate. Even when modules or cells are manufactured by the same supplier, the cells or modules used in a system may not be obtained from the same manufacturing lot of the supplier.
A third cause may be differential temperature of the cells or modules. Self-discharge rate for a cell or module is proportional to the cell's temperature, such that a higher temperature causes a cell or module to self-discharge faster. Cells or modules that experience different temperatures throughout operation may suffer different reductions in capacity. The different temperatures may be the result of proximity to other components in a device
Another cause may be differential power drain internal to a module. A battery management system may have component variations or a normal current leakage paths resulting in balancing of a module due to internal power drain of the module.
A fifth cause may be differential physical damage to a cell or module. Physical damage may reduce capacity or create internal shorts.
Another cause may be differential leakage of current internal or external to a cell or cells within a module. Differential leakage may be intermittent or continuous and may or may not cause a detectable amount of heat. Pack manufacturing defects are one cause for external differential leakage current. Cell defects or damage are another cause for external differential leakage current. For example, internal cell shorts may occur as a result of anode dendrites, separator dendrites, manufacture metal flake defects, weld splash defects, cell crush damage, excessive cell vibration, and/or shock. These internal shorts may generate localized heat within the interior of a cell.
Even small intermittent shorts may cause lasting damage to a battery cell. Although small intermittent shorts may burn themselves out, their occurrence may weaken the interior of the cell, which leads to more frequent and more severe internal shorts. Over time, and with stresses of charging and discharging, multiple instances of internal shorts may eventually result in an internal short severe enough to cause a thermal run away event. A thermal run away event is the release of a dangerous amount of energy that may destroy the cell within a very short time. The released energy may damage equipment or injure operators.
One conventional solution for monitoring for internal shorts or failure of the battery cells is a temperature monitor placed on an exterior surface or nearby to each battery cell being monitored. However, internal shorts often occur without causing a detectable temperature increase around the battery cell. Even though the temperature does not immediately rise, a thermal runaway event may nonetheless be building in the battery cell. During a future charge cycle of the battery cells, battery cells coupled to the internally shorted battery cell may overcharge, increasing the likelihood of a thermal runaway event due to the additional stress on the otherwise-healthy battery cells. The combination of the stressed healthy battery cells and the heat from the internally shorted battery cell may result in a thermal runaway event occurring that was originally undetectable by an external temperature sensor.